The advantages of digital technologies over analog are well known. The discrete nature of digital signals allows unambiguous interpretation and exact regeneration, totally eliminating the effects of noise and tolerance stackup within broad bounds. This tolerance to component variation has made it possible to manufacture extremely complex systems containing millions of elements on a single digital integrated circuit.
Systems which are inherently digital, or defined as such, require digital implementation and naturally benefit from the digital technology trends. Virtually all new systems being defined today are digital.
Systems which are inherently analog in nature can also be realized digitally. The analog signal must first be converted to a digital representation by sampling the signal at discrete instances in time and then quantizing the resultant samples to a discrete set of values. Although the digitization process itself cannot be perfect, the signal degradation due to digitizing can be made sufficiently small by sampling sufficiently fast and quantizing sufficiently finely. Once digitized, all operations can be performed arithmetically to any desired degree of precision, thus preserving the available fidelity of the original signal. The digital signal can be converted back to analog as needed.
In audio or radio systems, it is not possible. to completely eliminate the need for analog circuitry. The analog signal path is still required up to the point that conversion to digital is performed. The analog circuitry can be minimized by converting to digital as close to the analog signal source as possible, but the analog circuitry cannot be completely eliminated, forcing a mixed signal mode design. The analog-to-digital converter (ADC) itself is a mixed signal component.
There does exist a basic incompatibility between low level analog and digital circuitry. The switching nature of digital logic tends to generate significant electrical noise which usually does not interfere with the normal operation of other nearby digital circuitry. However this noise can be a problem in mixed signal designs due to the sensitive nature of analog circuitry to external interference. A sufficient level of isolation between the analog and digital sections can usually be achieved but is often challenging and may require iterated design. This is already experienced in automotive analog radio circuits having digital control inputs. In addition, although it is possible to accommodate both analog and digital circuitry on a single integrated circuit, most IC fabrication processes are not optimized well for both and may require additional processing steps.
Despite the many advantages, the transition from mostly-analog to mostly-digital of inherently analog systems has been slow although there is a definite trend and desire to do so. The primary disadvantage has always been cost. Replacing a few precision components with thousands of simple transistors has not proven to be cost effective but this is rapidly changing with continued advances in digital integrated circuit technology. As the economics of digital become more favorable, digital implementations of inherently analog systems will continue to emerge.
Traditional mobile radios are not architected appropriately to benefit well from the digital technology trends. The nature of the analog signal processing does not allow flexible partitioning of the signal path and almost necessitates that the entire radio electronics be contained within a single package. The inflexible nature of analog circuitry and the potential for interference has provided little incentive to integrate further which has resulted in a paradigm that separate radio receivers and/or transceivers must be designed and packaged independently. As the need for mobile communications has increased, this has resulted in several radio transceiver packages scattered throughout the vehicle, e.g., AM/FM radio, cellular phones, global positioning systems (GPS) and digital audio broadcast (DAB).